The global positioning system (GPS) has fundamentally changed the methods of navigation, location tracking, and time synchronization worldwide. With thirty-two satellites on orbit, the GPS provides continuous positioning service at almost anyplace signals can be received. With the advent of low-cost positioning sensors using GPS, accurate to a few meters, there has been a proliferation of the technology into core infrastructures including power systems, communications, transportation, and military. The importance of this capability as a national asset cannot be overstated and is highlighted by the fact that many other nations are now either operating or developing their own GNSS, including Russia, Japan, China and the European Union.
Despite its many advantages, GNSS has one significant drawback: satellite-based navigation systems signals are typically very weak as they reach the positioning receiver. In some cases, like the GPS, this is a key part of its design, but practically it is difficult to operate high power transmitters on orbit. These weak signals make it difficult to operate positioning receivers in obstructed environments, such as indoors, as the obstructions will tend to attenuate the signal power and render it useless for positioning or, at the very least, substantially degrade the overall measurement capability.
While significant effort has been made to overcome these limitations, particularly Assisted GPS and High-Sensitivity GPS, in practical terms meter level positioning in obstructed environments using GNSS is not feasible for broad usage. To provide positioning in obstructed environment another class of positioning technologies has been developed known as real time locating systems (RTLS), which derive from radio frequency identification (RFID) technologies.
Using a variety of ranging methods, such as time difference of arrival (TDOA), Received Signal Strength (RSS), fixed reader, and landmark tagging, RTLS offers a variety of positioning capabilities and accuracies. The most advanced and versatile systems tend to use TDOA and can offer positioning accuracy to within a few meters. Some of the systems even claim sub-meter accuracy, though this tends to be in highly controlled environments.
While promising, RTLS systems are very expensive to install and operate. When high accuracy is needed, the cost and complexity of the equipment can make it all but impractical except for a few limited applications. RTLS offers a variety of solutions that can be tailored to fit a variety of applications; however, when compared to the relative simplicity and wide availability of GNSS based positioning they all are less than desirable.
Further, for combined applications requiring positioning in both local area obstructed and wide area unobstructed environments, options are extremely limited as neither GNSS nor RTLS can satisfy the requirement alone. Combined RTLS and GNSS systems are impractical due to the fact that they are largely incompatible and are difficult to integrate and, as a result, very expensive. Several attempts have been made to adapt commodity GPS receiver technologies using pseudolites to provide RTLS capabilities. While attractive in concept, these solutions are at best too expensive and power intensive to be practical in addressing many of the RTLS applications and at worst they are illegal to operate in much of the world as they tend to jam normal GPS operations.
Accordingly, there is a need for a cost effective, highly accurate positioning technology that operates equally well in obstructed environments using locally deployed beacon reference points and can utilize GNSS reference points such as a GPS satellite for wide area unobstructed environments.